The present invention relates to a method for producing a cast body having a thin portion, which is used as a mold for molding a tire, a cooling fin in a cylinder block of motor cycle, or the like.
Since a cast body using an iron-group material such as iron and steel has high strength, such cast bodies are used for various purposes, such as molds for molding tires. Incidentally, the term xe2x80x9cironxe2x80x9d means an iron-group material containing more than 0.3 wt % of carbon, and the term xe2x80x9csteelxe2x80x9d means an iron-group material containing 0.3 wt % or lower of carbon. General methods for producing a cast body using iron-group materials such as iron and steel are sand mold casting, the lost wax process, and the Shaw process.
In sand mold casting, as shown in FIGS. 2(a) and 2(b), molten metal 3 of iron or steel is poured into a mold 2 (FIG. 2(a)) made of sand from a prototype 1 and subjected to casting.
In the lost wax process, as shown in FIGS. 3(a), 3(b), 3(c), and 3(d), a prototype 1 made of wax is immersed in slurry 4 containing a binder such as ethyl silicate and Substitute Specification refractory fine powder (FIG. 3(a)), a sand layer 5 is formed on the outer surface of the prototype 1 (FIG. 3(b)), the prototype 1 is heated to let the wax flow out to obtain a mold 2 of sand (FIG. 3(c)), and molten metal is poured into the mold 2 for casting (FIG. 3(d)).
In the Shaw process, molten metal of iron or steel is poured into a ceramic mold. Specifically, as shown in FIG. 4(a), slurry 4 having as main components ethyl silicate and refractory fine powder is poured into a prototype 1 made of rubber or the like to be gelated. Then, the ceramic mold is subjected to primary firing by a direct fire, and then secondary firing in an electric furnace to obtain a mold. Finally, as shown in FIG. 4(b), molten metal 3 of iron or steel is poured into the mold 2 for casting. Incidentally, primary firing by a direct fire is for forming fine cracks in the whole mold by rapidly evaporating alcohol contained in the compact. The cracks have the function of improving air-permeability upon casting and suppressing breakage of the mold.
In the Shaw process, a die-rapped compact is immersed in liquid having low viscosity and volatility at room temperature such as methanol, ethanol, and propanol in some cases, and left for 10-40 minutes for a stabilizing treatment. This is to fire the mold after the dimensions of the mold are stabilized by being left for a certain time after it is die-rapped, since the dimensional change during firing is large if the die-rapped compact is immediately fired. The reason why the compact is immersed in liquid such as ethanol in the stabilizing treatment is because the aforementioned cracks are effectively formed upon firing the compact by preventing the compact from the evaporation of alcohol therein.
However, in the above conventional method, the following problems arise for a cast body having a thin portion 6 having a thickness of 2 mm, e.g., a mold 7 for molding a tire having a protrusion 9 corresponding to a sipe (FIG. 5(a)), and a cylinder block 8 having a cooling fin for automobile (FIG. 5(b)).
First, even if a narrow sipe is formed in the mold, molten metal does not flow down to the bottom of the sipe upon molding, and therefore, formation of a thin portion is difficult.
Even if the thin portion can be formed, the strength properties, such as fracture strength, elongation, and tenacity, are insufficient in some cases depending on thickness of the thin portion. In the case of a mold for molding a tire, the thin portion easily breaks, for example, when rubber adhered to the mold is removed by sandblast.
Further, often in sand mold casting and the Shaw process, a mold is produced using a rubber prototype. However, the protruding portion corresponding to a thin portion is torn off when the mold is removed from the prototype, and thereby the prototype cannot be reused. In other cases, the protruding portion corresponding to a thin portion is deformed when a mold is produced, and therefore it is difficult to produce a mold having high dimensional precision. Further, in the Shaw process, even if a sipe corresponding a thin portion is formed in a compact of a mold, the fire does not reach up to the innermost recess upon primary firing of the compact, and the surface portion of the compact is rapidly heated. As a result, the width of the sipe is partially changed, and therefore molten metal is not carried well through the portion of the sipe having a narrow width. This causes the formation of a pore in the thin portion or unevenness in thickness of the thin portion (defect of insufficient run).
To solve the above problems, a method could be used in which a thin member 10 of iron, steel, or nickel alloy having the same thickness as the thin portion is disposed in a suitable portion of a mold 2 (FIG. 6(a)), and the thin member 10 is subjected to metal-ceramic insertion (FIG. 6(b)) to form a thin portion 6 in a cast body 11 FIG. 6(c)). However, microelements such as C, Si, and Mg are shifted from a matrix 12 to the surface or the vicinity of the surface of the thin member 10, and since this causes the following problems, the method is not practical.
Since the matrix of the cast body has lower melting point than the thin member originally, and the melting point of the thin member descends by shifting of microelements, the thin member melts if molten metal forming the matrix contacts the thin member, and thereby the composition in the vicinity of the thin member is locally shifted by components eluted from the molten thin member. Since the melting point in the portion where the composition is locally shifted descends, the portion is more slowly solidified than the other portion of the matrix. Therefore, replenishment for the shrinkage by the solidification of this portion is not received, and a pore 13 is formed in this portion (local shrinkage cavity).
Further, since components eluted from a molten thin member cause extraordinary curing in the vicinity of the thin member, the corresponding portion of a cast body becomes brittle. Therefore, the cast body is prone to having a low degree of impact load, or machining becomes difficult because of blade damage where machining is required after casting is complete.
The present invention has been made in view of such problems and aims to provide a method for producing a cast body having a thin portion, in which the thin portion has high dimensional precision and can be formed by casting. The present invention also provides a method for producing a cast body having a thin portion which does not create local shrinkage cavities and cause extraordinary curing, even if metal-ceramic insertion is performed, and which produces a thin portion having sufficient strength and high dimensional precision.
According to the present invention, a method for producing a cast body having a thin portion is provided, including the step of subjecting a thin member formed by one material selected from the group consisting of iron, copper, and nickel alloy, to metal-ceramic insertion in a matrix of iron or copper. The thin member includes a ceramic layer on the surface thereof to prevent the thin member from melting upon being subjected to metal-ceramic insertion.
According to another embodiment of the present invention, a method for producing a cast body having a thin portion is provided, including the step of subjecting a thin member formed by one material selected from the group consisting of iron, copper, and nickel alloy, to metal-ceramic insertion in a matrix of iron or copper, wherein a ceramic layer being formed, by the use of the thin member containing 0.5 wt % or more in total of at least one element selected from the group consisting of Al, Ti, Be and Mg, at least on or near a surface of the thin member at the time metal-ceramic insertion or at the time of heating prior to metal-ceramic insertion to prevent the thin portion from melting upon metal-ceramic insertion.
According to yet another embodiment of the present invention, any of the aforementioned methods for producing a cast body having a thin portion based on the Shaw process can be used, further comprising the steps of preparing a compact for a ceramic mold from a prototype, subjecting the compact to a stabilizing treatment, firing the compact to obtain the ceramic mold, and obtaining a cast body by pouring molten metal of iron or copper in the ceramic mold. The ceramic mold having the thin portion member disposed therein is molded from a metallic piece a prototype having a thin member made of iron or copper in the step of preparing a compact, and a burr generated in the compact is removed in the step of subjecting the compact to a stabilizing treatment.
According to the present invention, there is further provided a method for producing a cast body having a thin portion, comprising the steps of pouring molten metal of iron or copper into a ceramic mold, wherein the ceramic mold is produced by firing a compact having a metallic piece having the same thickness as the thin portion disposed in a portion corresponding to a thin portion, and then removing the metallic piece.
The cast body having a thin portion may be a die for molding a tire, and said thin portion may be a protrusion for forming a sipe.